4-bromine-1,8-naphthalic anhydride as raw material through the imidization and substitution reaction can get 4-alkoxy-N-18 alkyl-1,8-naphthalimide fluorescent compounds. The yield is as indicators to discuss the factors impacting on the imidization and the imidization process was optimized further. The reaction temperature was 50 ℃. The reaction time was 3 h. The molar ratio of 4-bromine-1,8-naphthalene anhydride and 18 amines was 1∶1.08. The yield of 4-bromine-N-18 alkyl-1,8-naphthalimide is up to 81.6%. The infrared spectra of the intermediate product showed it agreed with the requirement. The fluorescence spectra of the target products 4-alkoxy-N-18 alkyl-1,8-naphthalimide was detected. The fluorescence spectra showed that these compounds had obvious emission peak in the range of 510～550 nm wavelength when the excitation wavelength was 464 nm. Target products in the lubricants of automotive air-conditioning systems have better solubility, and they give notable green or yellow-green fluorescent in ultraviolet light exposure.